Oil, grease and other fluids that leak from vehicles onto the road, along with road grit and melted road tar and lane marking paint accumulate on roads and highways over time. When it rains this accumulated residue is sprayed onto the paint surface using water as a carrier system as oil and water do not mix. The highest concentration of road traffic film accumulates on your wheels, tyres and lower body panels.
When rain and surface water washes away all the water soluble components of traffic film contaminants, what remains are just the non-water soluble parts on the vehicle’s paintwork. Ultra violet (UV) radiation adheres and then cures these contaminants onto the surface. Most of the particulates are either metallic, or contain a metallic element and bear a positive electrical charge. Conversely, the exterior of most vehicles are negatively charged. Hence, these traffic film contaminants have a strong magnetic attraction to each other
Washing your car will remove most accumulated soiling, but normal car washing won't remove road traffic film (RTF). Auto paint is porous and this film will permeate and cause the paint to become opaque, dulling the colour.
Correction - the best way to remove RTF is by using a chemical paint cleaner, something slightly alkaline or a d-limonene solvent / detergent (P21S® Paintwork Cleanse) or for more stubborn staining Swissvax Cleaner Fluid Strong, which will remove any road film that normal car washing could not
Note: Be cognizant that some Traffic Film Remover products are meant for heavy commercial / construction type vehicle and are highly alkaline.
These commercial traffic film removers are usually used with high pressure washers and are formulated with high concentrations of hazardous solvents that may help to remove the dirt but can damage the vehicles paintwork over time